The present invention relates to a method for improving the adhesion of alkylated chlorosulfonated polyethylene (ACSM) and chlorosulfonated polyethylene (CSM) rubber compounds to a resorcinol-formaldehyde latex (RFL) treated polyester reinforcing element, and more particularly, this invention relates to the use of lower than conventional heat set temperatures to set a polyester cord for use in conjunction with an ACSM or CSM polymer system.
In bonding rubber to a reinforcing material in products such as power transmission belts and other reinforced rubber products, the common practice is to treat the material with a RFL latex dip to enhance the bonding of the rubber to the material. Typically, the RFL dip is prepared by mixing an RFL prepolymer with a latex. The RFL prepolymer provides adhesion to the reinforcing material and the latex provides adhesion to the rubber. Where the surface reactivity of the reinforcing material is low, as is the case with polyester cords, the cord is usually pre-treated with a composition which improves its reactivity. Compositions that have been used for this purpose include epoxy resins and isocyanates. Conventionally, belts are prepared by bonding a double-twisted cord to the rubber. The cord is immersed in a RFL dip, dipped into a neoprene latex dip, and transported through a vertical oven where it is heated under tension for a predetermined period of time. This causes the cord to elongate and restructure the polyester. This process is known as "stress elongation" or "heat set". The process dries the adhesives which are coated on the cord. The common RFL dip systems have also been developed for general purpose rubber compounds like natural rubber and styrene-butadiene-rubber. For these rubbers, the latex used in the RFL dip is often a terpolymer based on styrene, butadiene and vinylpyridine. For less reactive rubber compounds, like ACSM and CSM, the common RFL dip systems are not suitable.
In one conventional treatment for polyester cord, the polyester cord receives an initial polyphenylisocyanate treatment at 8 to 15 pounds of tension which is heat activated at a temperature in the range of 300.degree. F. to 425.degree. F. for 120 seconds to react the functional groups of the polyphenylisocyanate with the open bond sites in the cord. A RFL adhesive is coated over the reacted polyphenylisocyanate and dried, preferably in an oven, for 120 seconds at a temperature in the range of 180.degree. F. to 275.degree. F. to evaporate the water from the RFL and to keep the RFL from blistering prior to the heat setting of the cord. The heat setting of the polyester cord at 460.degree. F. for 60 seconds takes place after the adhesives are applied and therefore, the adhesives are subjected to this elevated temperature.
Normal polyester cord treatment temperatures are 470.degree.-475.degree. F. These temperatures are designed to achieve proper heat set and therefore, the optimum properties of the cord. These temperatures are near the melting point of the polyester cord which is in the range of 478.degree. F. to 490.degree. F. Experience has shown that the alignment of the optimum molecular structure in the polyester is achieved at 460.degree. F. at about 8 to 15 pounds of cord tension. Adhesion of polyester to ACSM and CSM rubber compounds is poor at the standard heat set temperature of 460.degree. F. Thus, there is a problem with using these standard heat set temperatures when bonding ACSM or CSM to a RFL-treated polyester cord. It has been found that the adhesion of the polyester cord to ACSM and CSM rubber compounds is greatly reduced at the standard heat set temperature of polyester cord because the RFL reacts vigorously with itself resulting in a crosslink density approaching a full state of cure leaving few, if any, sites available for bonding with the rubber.
Prior art methods of adhering CSM to polyester cord have included the use of neoprene latex and vinyl pyridine latex in the RFL latex dip.
U.S. Pat. No. 3,060,078 to Atwell teaches a method of bonding polyester cord fibers to a CSM rubber compound by treating the cord with a resorcinol-formaldehyde neoprene latex adhesive composition prior to heat setting the treated cord with the CSM rubber compound.
U.S. Pat. No. 3,325,333 to Kigane et al. teaches a method of adhering a polyester cord to a CSM rubber compound by treating the cord with an aqueous adhesive composition containing methylolated blocked organic polyisocyanate and a vulcanizable organic polymer at conventional or normal polyester treating temperatures.
Accordingly, there is a need for a method for improved adhesion between ACSM and CSM rubber compounds to a RFL-treated polyester cord at lower than conventional heat set temperatures.